Recently, the power supply develops toward high efficiency, high power density, high reliability and low cost the same as the most power manufactures. Especially for the purpose of energy saving and environment protection, lower and lower power loss is required when the power supply operates at light load or zero load condition.
The burst mode control method is more and more widely utilized as a power saving technique for the resonant converter of the power supply. For example, the burst mode control has been introduced into a flyback AC/DC converter of a notebook adapter and a buck DC/DC converter of a mobile phone.
In the burst mode control, when operating at light load or zero load condition, the switching mode power supply operates at the normal switching frequency fs for a period Ton and stops working for another period Toff, and then operates at the normal switching frequency again, wherein 1/(Ton+Toff) is always smaller than fs. And the power loss reduction ratio is Toff/(Ton+Toff).
From the above description, it is understood that the key point of the burst mode control is to generate a signal indicating the load status of the converter and the signal must be easy to be detected to activate the burst mode control. There are many methods in the prior art to detect the load status of the converter such as monitoring the secondary current of the transformer in the converter, or an intermediate voltage e.g. the feedback voltage in close-loop in the switching mode power supply.
The control method for using the close-loop feedback voltage as the intermediate voltage to trigger burst mode control is usually applied in boost type topologies, such as boost, buck boost and flyback, and in buck type topologies, e.g. buck, forward, half bridge and full bridge topologies. When the converter utilizing these topologies operates under light load condition, the operating current is discontinuous (DCM mode), which leads a significant change on the gain of output voltage to input voltage and the significant changes reflects in the sharp voltage variation in the compensation loop. Thus the load status of the converter can be accurately caught through sensing the feedback voltage in the compensation loop.
But it is a little bit difficult using the detecting method of the feedback voltage in the compensation loop in resonant converters. For example, for a resonant DC/DC converter 10 in the prior art as shown in FIG. 1, which is composed of a converting stage 11, a transformer Tx, a rectifying stage 12, and a filtering and load stage 13, wherein the converting stage 11 includes a resonant capacitor Cr, a resonant inductor Lr, and a magnetizing inductor Lm; the rectifying stage 12 includes diodes D1 and D2, and the filtering and load stage 13 includes a filtering capacitor Cout and a load Rload. The working principle of the resonant DC/DC converter 10 is to start with switching a DC voltage via the switches Q1 and Q2, then receiving a high-frequency carrier wave by the converting stage 11, following by rectifying the output of the converting stage 11 by the rectifying stage 12, and finally the output of the rectifying stage 12 is filtered by the filtering and load stage 13 which outputs to the load Rload.
When using the detecting method of the feedback voltage in the compensation loop in the resonant converter 10 as shown in FIG. 1, which is always controlled by FM (frequency method), the operation frequency doesn't change much between different loads. If the operation frequency doesn't have to vary much, the feedback voltage doesn't change much. So it is hard to implement the burst mode control by monitoring an intermediate voltage. The displacing program is to implement the burst mode control by monitoring the secondary current of the transformer Tx, but it is much complicated than the method described above.
In order to overcome the drawbacks in the prior art, a resonant converter and burst mode starting method thereof is provided. The particular design in the present invention not only solves the problems described above, but also is easy to be implemented. Thus, the invention has the utility for the industry.